Earth Clouds are masses of minute water droplets and / or ice crystals formed by the condensation of water vapour and held in suspension in the atmosphere. Condensation, which results from cooling, usually takes place around nuclei such as dust, smoke particles and salt. Such particles are called condensation nuclei.
Earth Clouds are of different types and they can be classified on the basis of their form and altitude.
On the basis of form, there are two major groups:
- Stratiform or layered types, and
- Cumuliform or massive types.
- These clouds, which are fairly thin and blanket like, are sub-divided into three main categories on the basis of altitude.
- High Clouds (mean ht 5-13 km)
- Cirrus Clouds: Indicates fair weather.
- Cirrocumulus Clouds: Forms the mackerel sky.
- Cirrostratus Clouds: Produces a halo around sun and moon.
- Middle Clouds (mean ht 2-7 km)
- Altocumulus Clouds: Indicate fine weather.
- Alto-stratus Clouds: Associated with development of bad weather.
- Low (mean ht up to 2 km)
- Stratus Clouds: Brigs dull weather, usually accompanied with a drizzle.
- Nimbostratus Clouds: If rain or snow is falling from a stratus cloud, it is called nimbostratus.
- Stratocumulus Clouds: Indicators of fair or clearing weather.
- They are massive clouds having a vertical extent from 1,500 to 9,000 m. They resemble the head of a cauliflower. When these clouds are sunlit, they are brilliantly white and are called ‘wool-clouds’. They occur mainly in summer and are produced by convection.
- Cumulonimbus Clouds: Under different weather conditions, a cumulus cloud may develop into cumulonimbus, the thunderstorm cloud mass of enormous size which brings heavy rainfall, thunder and lightning and gusty winds.
- It refers to falling of water, snow or hail from the clouds and results when condensation is occurring rapidly within a cloud.
- The most common form of precipitation is rain and it is formed when many cloud droplets coalesce into drops too large to remain suspended in the air. Rainfall occurs when the dew point of air is above the freezing point.
- Sometimes the raindrops freeze before reaching the ground and precipitation occurs in the form of ice pellets, called sleet.
- Snow is produced when condensation takes place at a temperature below freezing point, so that the minute crystals (spicules) of ice form directly from the water vapour.
- Hail consists of masses of ice with a layered structure. It occurs when there are very strong updrafts in the clouds carrying raindrops up to a high altitude, causing them to freeze. Hail stone is a rounded lump of ice having concentric layers.
Conditions for Precipitation
- There are three possible ways by which precipitation is produced.
- It is caused by heating of moist air in the lower layers of atmosphere which rises, expands, and is cooled adiabatically to its dew point. Convection rain is often accompanied by lightening and thunder. It occurs in regions near the equator in the afternoon as a result of the constant high temperature and high humidity.
- In this, precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises in the windward side, it is cooled at the adiabatic rate. If sufficiently cooled, precipitation results; when the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rain shadow, may exist on the leeward side.
Cyclonic frontal Precipitation
- When the air is caused to rise upwards due to cyclonic circulation, the resulting precipitation id said to be of the cyclonic type.
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